Heretofore, along with high integration of a circuit board to be utilized for e.g. a powder module, heat generated by a semiconductor device tends to increase. In order that the heat be effectively dissipated, various methods have been studied, and ceramics such as alumina, beryllia, silicon nitride and aluminum nitride have been utilized. Among them, aluminum nitride is a suitable material in view of high thermal conductivity, high insulating properties, harmfulness, etc., and in addition, it has attracted attention since it has plasma resistance and a coefficient of thermal expansion close to that of silicon in recent years, and has been used as a jig for a semiconductor production apparatus in the form of a single substance, as embedded in a metal heater, as fixed to a metal, etc. In any of these application forms, an aluminum nitride sintered body having a high degree of parallelization and being less likely to be warped has been desired. In order to obtain such an aluminum nitride sintered body, it is important to produce an aluminum nitride sintered body having small sintering shrinkage. Here, the sintering shrinkage means a phenomenon such that the dimensions of the sintered body after sintering are smaller than the dimensions of the molded body before sintering, and the sintering shrinkage becomes necessarily small when a high filing rate of a powder can be achieved and the density of the molded body before sintering can be increased.
Heretofore, for production of an aluminum nitride powder for production of an aluminum nitride sintered body, an alumina reduction method and a method of direct nitriding of a metal aluminum powder have been commonly employed, but these methods have both merits and demerits. An aluminum nitride powder obtainable by the alumina reduction method has a uniform particle size and a small oxygen amount as compared with the direct nitriding method, whereby it is easily sintered to produce a sintered body having a high thermal conductivity, but the shrinkage factor at the time of sintering tends to be large, warpage or deformation is likely to occur, and the production cost tends to be high. On the other hand, in the direct nitriding method, the aluminum nitride powder will be easily produced at a low cost, but since the method comprises a grinding step, the obtained aluminum nitride powder tends to contain an increased amount of impurities such as oxygen and the thermal conductivity can hardly be higher than that achieved by the alumina reduction method. Further, neither of the aluminum nitride powders obtained by these production methods has been able to sufficiently achieve both higher thermal conductivity of the aluminum nitride sintered body and reduction in the shrinkage factor at the time of sintering.
The present applicant has proposed (Patent Document 1) that for production of an aluminum nitride sintered body which achieves both high thermal conductivity and small sintering shrinkage, an aluminum nitride powder having a specific particle size and a specific oxygen amount may be used, and that such an aluminum nitride powder can be prepared by producing several types of aluminum nitride powders differing in the oxygen amount and the particle size and suitably combining them.
Patent Document 1: Japanese Patent No. 3403500